This invention relates to a delivery system and method of a cryogenic fluid, such as liquefied natural gas (LNG), from a storage tank to a use device, such as a natural gas powered vehicle engine. An embodiment of the system of the invention is particularly suited for markets in which pre-saturation of the LNG fuel is not performed, though it may also function as a source of “trim heat” if the tank pressure falls below a pre-defined level.
Many heavy-duty vehicle engines require that the intake pressure of natural gas be around 100 psig. In most markets, LNG is saturated, or heat is added, to a point at which its vapor pressure is roughly equal to the pressure required by the use device. This process of building saturation pressure is typically performed at LNG fueling stations. However, there exist some markets in which this saturation of the fuel before transferring it to the vehicle storage tank is not performed or is not performed to an extent great enough to achieve 100 psig saturated liquid in the vehicle tank after fueling. Thus, the storage tank may end up being filled with LNG well below the desired pressure.
One proposed method for building tank pressure is to utilize a pressure building circuit that is common on many stationary cryogenic cylinders. These circuits function by utilizing gravity to feed liquid cryogen into a vaporizer. Upon vaporization of the liquid, its volume expands and the evolved gas is routed to the vapor space above the cryogen, building a head of vapor pressure above the liquid phase in the tank. However, there are three distinct problems with this type of circuit for LNG vehicle tanks. First, as most LNG vehicle tanks are mounted horizontally, there is small liquid head pressure compared to a vertical tank to force liquid into the vaporizer. Second, since LNG vehicle tanks are used in mobile applications, any vapor pressure that is built above the liquid phase will quickly collapse as soon as the vehicle is in motion and the liquid and vapor phases mix. It may take several hours or more to add enough heat in this fashion to fully saturate the bulk of LNG in the tank. Third, because pressure building coils are gravity feed systems, the feed line is directly connected to the liquid space. In a vehicle accident, this open liquid line can be damaged, creating a fire hazard due to the large volumes of gas generated from a liquid leak.
Another proposed solution is referenced in U.S. Pat. No. 5,163,409 to Gustafson et at whereby compressed natural gas (CNG) is used to add vapor pressure above LNG to deliver the fuel at an elevated pressure. However, this solution requires a second tank for CNG be mounted on the vehicle, which would add weight and occupy space on the vehicle chassis.